Mention EOL Elasticsearch 6.6 in 3.x release notes

Initialize notes with this for the next Gerrit 3.0 and 3.1 minor
releases. For the next Gerrit 3.2 minor release, add this to the already
initiated notes.

Change-Id: I262dc4aa21bb39d3bc9a6dde5adaee3814cf20b9
3 files changed